Hot water boilers are devices that supply water by heating and may be divided into domestic and industrial types according to the purpose of use. Generally, industrial boilers may be used in industrial facilities such as factories and large-scale residential facilities. Accordingly, such industrial boilers are required to supply high-temperature hot water or steam in large quantities, and thus, they are required to have high capacity and high efficiency.
Such large capacity hot water boilers may be divided into a water tube type boiler in which water flowing along a plurality of water tubes connecting vertically arranged headers absorbs heat from gas burned by a burner to become hot water; a smoke tube type boiler in which water contained in a main body forming a water tank absorbs heat from combustion gas passing through a plurality of smoke tubes passing through the inside of the main body to become hot water; and a combined boiler combining the water tube type boiler and smoke tube type boiler, according to the hot water production methods. Among these, the combined boiler has both the characteristics of the water tube boiler and the smoke tube type boiler and exhibits an advantage of excellent thermal efficiency.
The combined boilers are used to heat large residential facilities in some district heating energy facilities in Korea.
FIG. 1 shows a partial cross-sectional view schematically showing a conventional combined hot water boiler.
Referring to FIG. 1, the conventional combined hot water boiler may include a water tube unit 1 and a smoke tube unit 2 arranged side by side, and a connection unit 3 connecting them at the bottom of the water tube unit 1 and the smoke tube unit 2. Here, the connection unit 3 allows the water tube unit 1 and the smoke tube unit 2 to communicate with each other.
The water tube unit 1 may include an upper header 1a, a lower header 1e, a combustion chamber 1c disposed between the upper header 1a and the lower header 1e, and a plurality of water tubes 1b which connects the upper header 1a and the lower header 1e and is provided in the combustion chamber 1c. A burner 4 installed on the upper header 1a may generate a flame downward toward the combustion chamber 1c provided with the water tubes 1b, and the combustion gas thus generated may be moved to the smoke tube unit 2 through a post-combustion chamber 3c of the connection unit 3. The combustion gas transferred to the smoke tube unit 2 heats cold water (circulation water) supplied into a main body 2a of the smoke tube unit 2 while moving upward along a plurality of smoke tubes 2b extending in the longitudinal direction in the main body 2a and is then discharged to an exhaust duct 5 provided at an upper part of the main body 2a. 
The circulation water heated by the combustion gas in the main body 2a of the smoke tube unit 2 is further heated by sequentially passing through a plurality of connecting water tubes 3b connected to the bottom 2c of the main body 2, a header 3a of the connection unit 3 and the water tubes 1b of the water tube unit 1, and the further heated water is then supplied to a place needing the hot water through an outlet 1d provided at the upper header 1a of the water tube unit 1. Accordingly, the high efficiency of the boiler may be achieved by such hot water supply method.
The combined hot water boiler, as shown in FIG. 1, may be referred to as a stand type hot water boiler with a combined water tube/smoke tube since the water tubes 1b and the smoke tubes 2b are formed by extending in a longitudinal direction, that is, in the direction of gravity.
However, in the conventional combined hot water boilers, since after discharged through the outlet 1d and circulated through a predetermined path, the cold water returning into the main body 2a of the smoke tube unit 2 through the circulation water port 2d is discharged near the top of the smoke tube 2b for effective heat exchange, the following problems may entail.
First, since the heated water in the main body 2a of the smoke tube unit 2, particularly the water heated from the lower side, is transferred to the upper part by convection, a flowing collision phenomenon occurs between the cold water flowing through the circulation water port 2d and the heated water moving to the upper part. Therefore, the cold water flowing through the circulation water port 2d cannot move smoothly to the lower side of the main body 2a. In addition, since there is a phenomenon in which the heated water is stagnated at the central portion of the main body 2a and thus relatively less heated water is positioned at the edge portion of the main body 2a in which the connecting water tubes 3b are arranged. As a result, the cold water flowing into the main body 2a is not heated sufficiently but is supplied to the connecting water tube 3b and the water tube 1b. Accordingly, this not only lowers the thermal efficiency of the boiler, but also affects the reliability of the heating system using these boilers.
In addition, due to the above phenomenon, heat exchange cannot be performed smoothly at the lower junction of the smoke tube 2b into which the combustion gas of high temperature is introduced. Therefore, the damage around the smoke tube 2b frequently occurs due to severe thermal shock. Specifically, the temperature of the combustion gas generated in the burner 4 is about 1,100 degrees Celsius. Since however the cold water introduced into the main body 2a cannot smoothly move to the lower portion of the main body 2a, the heat transfer from the combustion gas to the cold water is not performed sufficiently. As a result, a large thermal load is applied to the bottom portion of the main body 2a, that is, the bottom portion of the smoke tube 2b and the bottom portion 2c of the main body 2a. 
As shown in FIG. 2, since the smoke tube 2b is jointed to the bottom 2c of the main body 2a by welding, the joint portion is relatively weaker than the other parts. As described above, if the thermal load is continuously applied to the joint portion of the smoke tube 2b and the bottom 2c of the main body 2a, cracks can easily occur on the joint portion. If such cracks occur continuously during the operation of the boiler, the joint portion is damaged, and as a result, water in the main body 2a of the smoke tube unit 2 may leak, as shown in FIG. 3. If it is left unattended, it may be a serious threat to the safety of the boiler. Therefore, there are problems that maintenance work such as replacing the smoke tube 2b is inevitable, the maintenance cost of the boiler is excessively high, and the life of the boiler is shortened and its stability is not guaranteed.